Because the cast of First Class didn't so well at the box-office compare to the first four X-Men films? Money matters the most to studios.

Fan-boys will complain about the roles for most of the characters especially like Storm, Cyclops and Rogue but most of the general audience couldn't care less if Wolverine camera hogs and those three aren't close to the comics. People just want to see a good and entertaining film.

We could only hope that Bryan Singer will give those characters more justice since this is the third X-Men movie that he's directing.